wget calls cron.php up to 20 times in a row

If cron.php does not return because of a script timeout, wget calls by default the site up to 20 times total until it gets a valid response.

When cron times out, the 20 times default-retry mostly aggravates things. A typical symptom of those retries are watchdog messages from cron in an unexpected ~ 1 minute interval.

This tiny patch adds the -t flag to the suggested wget call in INSTALL.txt to make sure that wget only calls cron.php once in a script execution.

 -t number
 --tries=number
     Set number of retries to number.  Specify 0 or inf for infinite
     retrying.  The default is to retry 20 times, with the exception of
     fatal errors like ``connection refused'' or ``not found'' (404),
     which are not retried.